The present invention relates to a new and improved method of cooling a cast strand which is forming in an oscillating mold during the continuous casting of metals, typically steel, wherein the strand, during its passage through the mold, is indirectly cooled in a first zone as well as guided at all sides, and in a last zone the cast strand is guided by substantially strip-shaped support surfaces extending essentially in the direction of travel of the strand as well as directly cooled by water guided in a substantially strip-like configuration along the strand. The invention further pertains to an improved construction of continuous casting mold or mold arrangement for the performance of the aforesaid method.
During the continuous casting of steel, especially at high casting speeds, it is extremely important that there be produced as uniform and as thick as possible strand shell or skin upon departure of the cast strand or casting out of the mold.
Due to the contraction of the strand shell within the mold such lifts off of the mold walls or, depending upon the cross-section of the strand and the taper of the hollow mold compartment, produces an irregular contact of the strand with the walls of the mold viewed over the periphery of the strand. Due to this irregular contact of the strand with the mold walls there is formed, especially at the lower portion of the mold, a strand shell which is of varying thickness at the outlet end of the mold and possesses the well known drawbacks, such as for instance diamond profile, fissures, metal breakouts and so forth.
Hence, for the purpose of producing strands having a uniform thickness of the shell over the periphery of the strand at the outlet end of the mold it therefore was beneficial to use short molds equipped with a subsequently arranged spray cooling device. However, the thickness of the strand shell at such mold is thin at the time that the strand departs from the mold and thus severely prone to the metal breakout phenomena, so that even the slightest defects at the strand shell can precipitate metal breakout. Hence, from the standpoint of counteracting metal breakout long molds are preferred. Yet such long molds possess at the lower region thereof an extremely small cooling capacity or efficiency, and considered with regard to the periphery of the mold, an irregular cooling efficiency. Thus, such molds are not suitable for producing uniformly thick strand shells and equally not suitable for high-speed casting operations where there should prevail safeguards against metal breakout, especially when casting billet- and bloom-cross-sections.
There is further known to the art an oscillating mold which, viewed in the direction of travel of the strand, is equipped with two different successively arranged cooling devices i.e., cooling devices working according to different cooling techniques. The first cooling device consists of cooled walls which delimit the hollow mold compartment or mold cavity. These mold walls guide the strand and indirectly cool the same. Directly following such first cooling device is the second cooling device which guides the strand by means of strip-shaped guide surfaces arranged in the direction of strand travel and the strand is directly cooled be means of strip-shaped cooling water channels located between these guide surfaces. However, this prior art mold construction is associated with the drawback that the vapor which forms in the strip-shaped cooling water can ascend at the shrinkage gap between the mold walls and the strand at the indirectly cooled mold portion up to the level of the molten bath and thus can cause explosions.
Furthermore, there is also known to the art a mold equipped with an indirect cooling arrangement and a strip-shaped direct cooling arrangement which is located after the indirect cooling arrangement in the direction of strand travel. Strip-shaped guide surfaces are provided between cooling water channels. The lower portion of the mold, which is equipped with the direct cooling arrangement, consists of two oppositely situated pivotable cooling jaws, whereas both of the other sides form an extension of the indirect cooling arrangement. At a location above the cooling water channels both of the pivotable cooling jaws are equipped with two grooves which extend transversely with respect to the direction of travel of the strand, whereby, viewed in the direction of strand travel, the first groove serves to draw-off the water vapor and the second groove, which is in communication with the cooling water channels, serves for the introduction of air. Viewed over the periphery of the mold, this mold produces an irregular shell thickness at the strand because the cooling effect at walls which abut one another is markedly different. Furthermore, the pressed-in or introduced air causes pronounced scaling of the strand surface and all grooves extending transversely with respect to the direction of strand travel tend to rapidly clog with scale. The removal of water vapor by means of the thus provided grooves is no longer insured for and the water vapor ascends to the level of the bath and can cause explosions. With such molds it is therefore not possible to fulfill those requirements which prevail for continuous casting of steel at high casting speeds.